This application claims the priority of Germany Application No. 101 57 216.6-35, filed Nov. 22, 2001, the disclosure of which is expressly incorporated by reference herein.
The invention concerns a receiving array antenna for converting received signals from antenna elements into an intermediate frequency signal.
In active receiving array antennas, which usually consist of individual elements or subgroup elements, the received signals (radio frequency (RF) or high frequency signals (HF)) are converted to an intermediate frequency range (baseband). Digital or analog signal overlay, so-called “beam forming,” generate electronically controlled antenna characteristics in the intermediate frequency or baseband range. For this purpose, a conversion of the signals of the antenna elements or the subgroup elements takes place in a circuit including low-noise preamplifiers and frequency converters (mixer circuits). A local oscillator signal is therefore necessary for operating the mixer circuit and a calibration signal is necessary for the calibration of the entire circuit of the array antenna.
FIG. 1 shows a receiving array antenna with, for example, 4 channels according to the state of art. Each channel K include an antenna element ANT which is connected to a preamplifier, usually a low-noise preamplifier (LNA). A mixer MIX is connected downstream of the preamplifier LNA, and the output of the LNA is connected to the first input of the mixer MIX. The intermediate frequency ZF is applied and can be tapped at the output of the mixer MIX.
To calibrate the entire circuit, a calibration signal CAL is overlayed in each channel K on the received signal RF at the output of the antenna element ANT. This overlay usually takes place by means of a coupler KOP which can be, for example, a 3dB coupler. The calibration signal is generated centrally in a known circuit and is supplied by means of a distribution network VNK to the individual couplers KOP of the channels K.
To operate the mixers MIX, a local oscillator signal LO, which is generated centrally in a local oscillator, is supplied by another distribution network VNL to the mixers MIX of the individual channels K. The mixers MIX are therefore configured as so-called balanced mixers. The signal to be converted, that is, the received signal of the antenna element (ANT), also called useful signal, and the local oscillator signal LO can be supplied to the mixer MIX at two different gates.
The disadvantage of this interconnection is that the central local oscillator signal must be generated with a very high power when the number of channels is large. Another disadvantage is that, because of the two separate networks, a high interconnection complexity is necessary. This leads in particular in an antenna structure in planar technology to a construction of the antenna with a large number of printed circuit board planes (multilayer construction in strip line technology) [1]. Further disadvantages with respect to the production costs of the array antenna result from this.
It is therefore the object of the invention to disclose an active receiving array antenna with an interconnection which leads to a fundamental simplification of the antenna structure and which can be realized with a low quantity of printed circuit board planes.
In accordance with the invention, a common distribution network is provided for the central local oscillator and calibration signal, which is interconnected in such a way that the central oscillator and calibration signal are coupled into the circuit at the output of the receiving antenna element.
The weak received signal of the antenna element, the weak central calibration signal, and the stronger central local oscillator signal are therefore amplified in the preamplifier circuit of the invention. This provides the advantage that the level of the central local oscillator signal can be selected lower with respect to the state of the art, since the level of the local oscillator signal is increased in the circuit in accordance with the invention due to the preamplifier.
The amplification or the level of the central local oscillator signal is suitably selected so that the preamplifier does not go into saturation and therefore maintains its low noise factor. In this way, the amplification of the level of the central local oscillator signal brings about in particular a saturation of the stage connected downstream of the preamplifier through the preamplifier. This stage therefore shows non-linear properties, which, with a simultaneous presence of weak signals from the receiving antenna elements, a weak calibration signal, and the strong local oscillator signal, leads to the frequency conversion of the weak signals in the intermediate frequency range. The stage acts therefore as mixer with parametric mixing.
The mixer stage can furthermore be designed advantageously as an unbalanced mixer with additive mixing. Moreover, a bipolar or field effect transistor or a diode, for example, can be used in the mixer as parametrically controlled element. The advantage of an interconnection of the mixer as unbalanced mixer is that the useful signal and the local oscillator signal are supplied to the mixer at the same gate.
With the interconnection of the array antenna of the invention, it is possible to provide the local oscillator signal with a phase inversion between neighboring channels. This leads in particular to a partial cancellation of the signals emitted by the antenna when taking the undesirable emission of the local oscillator signal of the array antenna into consideration. Therefore, the emission of local oscillator signals can be reduced. This, however, can also be realized by additional filter elements in each antenna element itself.
The mixer can, however, also be configured advantageously as another amplifier stage, which can be operated as active mixer by means of operating voltages. In this way, the conversion behavior of the mixer can be optimized. The mixer can be practically interconnected with filter elements for the signal frequency (useful frequency) of the antenna elements, the local oscillator frequency, the intermediate frequency, and the image frequency [2].
A filter can be connected advantageously at the input of the mixer by means of which an image frequency signal generated in the circuit is filtered out, but instead the local oscillator and received signal are allowed to pass through. Furthermore, it is advantageously possible to connect another filter to the output of the mixer. By means of this filter it is possible to retrieve the intermediate frequency signal to the output of the mixer. The remaining signals are isolated by means of suitably selected impedances. The conversion behavior of the mixer is optimized in this way.
The filtering properties of the array antenna according to the invention can furthermore be expanded and used in the image frequency range for the suppression of the received power to realize a low noise sensitiveness and low (single sideband) noise factor of the receiving channels. The series connection of the amplifier stage and the mixer can also have a high attenuation of the intermediate frequency, whereby the reception of intermediate frequency signals can be suppressed.
Another advantage of the receiving array antenna of the invention is that, because of the common distribution network for the local oscillator and calibration signal, there can be a savings of printed circuit board planes when realizing the antenna structure in planar technology. The circuitry in accordance with the invention in the form of a series circuit of low-noise preamplifier and imbalanced mixer can result in a further savings of printed circuit board planes. This provides a further advantage with respect to decreased production costs.
The receiving array antenna of the invention can be used in receiving systems of satellite communication and radiometry, in particular because of the very low signal level and the very low signal dynamic. Other advantageous application fields are radio networks, for example, wireless LAN (local area network), mobile network with Space Division Multiple Access/SDMA technology (for example, smart antennas) as well as in radar sensors.